1. Field of the Invention
The present invention relates to non-impact printing apparatus using a multiple number of electro-optical recording elements, such as an LED printhead used for recording in an electrophotographic system.
2. Description Relative to the Prior Art
In the prior art as exemplified by U.S. Pat. No. 4,435,064, light-emitting diode (LED) printheads are known for use in recording on electrophotographic receptors such as photoconductive drums or webs. By selective enablement of the LED's, images may be recorded, developed, and subsequently transferred to a copy sheet such as plain paper.
As the LED's are known to be lambertian light emitters, it is known to provide a lens to focus light from the LED's onto the recording surface. This allows the LED's to be moved away from direct engagement with the recording medium. One lens that may be used is known as a Selfoc lens, which is a linear array of gradient indexed optical fibers. The lens is positioned between the recording medium and the LED printhead. A problem with the use of LED arrays is that the arrays are each formed in chips of 64, 96 or 128 LED's and then must be butted together end-to-end to provide a single row of several thousand LED's. While the LED to LED spacing remains uniform between LED's on the same chip, it can vary for adjacent chips at the ends of butting arrays. Furthermore, the spacing between chips on the same array chip can be relatively large. The LED printheads are used in electrophotographic systems to erase charge from the photoconductor, problems can arise due to the inability of the charge to be uniformly erased. The result can be streaks in the background (pos/pos) systems or distorted characters and lines (neg/pos) systems or a combination of both deleterious effects can be seen in either system.
It will be understood that a perfect optical image of the LED projected to the photoconductor is undesirable because the LED light sources in the array are not contiguous. The gaps in the array would leave white or black gaps in the developed image. Overlap of the light from the LED's in the array is required to achieve coverage of the full area of the photoconductor.
In order to overcome this problem, the shape of the LED's may be changed and/or the images thereof varied by moving the LED's slightly from the object plane or by moving the lens. While this defocussing approach can be satisfactory and is commonly used, there are problems associated with same since devices for moving either array must do so without moving one end more than the other. Additionally, the shape of the LED elements may not be best suited for the printing application described above. In addition, a significant defocussed position of the Selfoc lens can form multiple images of a picture element instead of a single image.
It is therefore one object of the invention to provide an improved printhead which overcomes the disadvantages of the prior art.